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7. Fault finding
7.1 Preliminary procedure
Before undertaking the following fault finding procedure, ensure that the valve has been
isolated and that upstream and downstream pressures are zero. Possible fault checks are
given in a logical order below.
7.2 Downstream pressure zero or too low
If downstream pressure drops below set pressure or is zero, check the following:
1. No high pressure steam reaching the pressure reducing valve. Check that steam is
turned on and that the strainer is clear. (To assist with commissioning and fault finding
it is recommended to fit an upstream pressure gauge).
2. Pressure adjustment spring broken.
3. Pipe assembly (Item 'P' on page 31) blocked. Remove by uncoupling the union nuts and
blow through to clear obstruction.
4. Control orifice blocked. Unscrew from the outlet port and clear out obstruction. For
identification this coupling has a groove around the hex.
5. Main diaphragms fractured. Replace the diaphragms following Steps 36 to 43.
6. Pilot valve plunger too short. Check as Section 5 Step 13.
7. Valve capacity insufficient for downstream conditions.
a) Check that the upstream pressure is correct. If it is low, the capacity of the valve
will be reduced.
b) Ensure that the pressure control pipe is fitted as recommended in Section 3.8 and
if necessary fit an external pressure control pipe as described. If the downstream
pressure is still too low, then a larger valve with more capacity is required.
7.3 Downstream pressure too high
If the pressure on the downstream side of the pressure reducing valve has risen above the
required set pressure check the following:
1. Downstream pressure control pipe blocked. Dismantle and blow through.
2. Control orifice (item 'P1' on page 31) blocked. Unscrew the pipework from the side of
the body and clean out. For identification this coupling has a groove around the hex.
3. Pilot valve diaphragms fractured. Check and replace (see page 31).
4. Pilot valve or pilot valve plunger is sticking. Follow Steps 7 to 11.
5. Main valve not seating. Check as page 31.
6. Main valve pushrod sticking. Follow Steps 36 to 43.
7. Pilot valve plunger too long. Check as Section 5 Step 13.
8. Pilot valve not seating. Refer to procedure on page 31.
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